Photolyses of the thiohydroximate ester derivatives 13 and 21 of the 4- (benzyl-seleno)-2-azetidinoines 7 and 20, afford the 1-aza-7-oxo-4-selenabicyclo- [3.2.0]heptane ring systems 14 and 21 in good to moderate yield in processes that presumably involve intramolecular homolytic substitution at selenium with expulsion of benzyl radical. Extension of this methodology to the preparation of derivatives 24 of 12,2a-dihydro-1H,8H-azeto[2,1-b][1,3]benzoselenazin-1-one (22) is also described.